The present invention relates to a system for heating an offshore pipeline. More particularly, the present invention relates to a system for providing direct electric heating along the length of an extended subsea pipeline. Heating is caused by a combination of electrical resistance and magnetic eddy current effects associated with transmission of an alternating current through the pipeline.
Offshore hydrocarbon recovery operations are increasingly pressing into deeper water and more remote locations. Here it is very expensive to provide surface facilities and it is desirable to minimize these requirements. Often satellite wells are completed subsea and are tied to remote platforms through extended subsea pipelines as a means to reduce the production cost. Even these platforms serving as central hubs in the offshore infrastructure are provided only the minimal facilities required for collecting and partially treating the well fluids before exporting them toward onshore facilities through yet more subsea pipelines. However the subsea pipelines crucial to this infrastructure prove a weak link as they are subject to plugging with hydrates or with paraffin deposition. Both hydrates and paraffins are of limited trouble at the pressures and temperatures experienced at the producing well, but can cause serious plugging problems when cooled to lower temperatures during pipeline transport.
Hydrates are the product of complicated chemistry in which water and certain hydrocarbon components of the produced well fluids combine to form ice-like crystals in pipelines as the temperature decreases during transport. The resulting hydrate crystals can suddenly solidify and plug the bore of the subsea pipeline. Paraffins are also a product of temperature in the pipeline and come out of suspension and deposit on the pipeline walls when the well fluids are below the "cloud point" which may be as high as 100-120 degrees Fahrenheit. Eventually this waxy buildup can completely seal off a pipeline.
These difficulties are combated between satellite subsea wells and platform hubs by insulating the pipelines and moving the produced well fluids as quickly as possible to minimize temperature loss. However, the long length of such pipelines renders passive insulation ineffective and it is often necessary to resort to large amounts of chemical inhibitors or to mechanical clearing operations to maintain the pipeline free of plugs.
In conventional practice, removal of a hydrate plug requires reducing the fluid pressure on both sides of the plug and applying chemical agents to the plug. Paraffin buildup is most often remedied by frequent routine pigging to scrape away the deposits fouling the bore of the pipeline. Before entering the pipelines between the platform and the onshore facilities, the fluids may be dewatered, separated into oil and gas, and treated with additives or other refined products. Again, it is often necessary to supplement this platform processing with routine pigging operations, even in these export pipelines.
Pipelines that are shut-in during workover of the wells or during work on the platform facilities are particularly susceptible to hydrate and paraffin problems as the hydrocarbon temperature drops toward the ambient seawater temperature. Thus, in present practice it is sometimes necessary to displace the hydrocarbons throughout an entire subsea pipeline with fluids that protect the pipeline during such operations. Further, it is then necessary to purge such fluids before production can resume. This is not an insignificant expense in both time and materials when considering pipelines whose lengths are measured in miles and tens of miles.
Thus, there is remains a clear need for an economical means to protect extended subsea pipelines from hydrate formation and paraffin deposition.